A subband coding method is a method of frequency-dividing an image signal, thereby to perform a coding process for signals (subband signals) in respective frequency bands. The subband coding is characterized in producing no block distortion in principle differently from a block-based orthogonal transform such as a discrete cosine transform, and in addition hereto, allowing hierarchy coding to be easily realized by recursively dividing a low-band component. The subband coding employing a wavelet transform for JPEG 2000, being the international coding standard, is adopted for a stationary image.
In a case of applying the subband coding for the moving image coding, not only a correlation in a spatial direction of signals, but also a correlation in a time direction thereof has to be taken into consideration. Conventionally, the sunband MC (Motion Compensation) has been studied of, after performing a subband division for a raw signal, removing the time-directional correlation by performing a motion compensation foe each subband region. However, there exists in the subband MC the problem that a predictive efficiency is bad in the high-band subband and a coding performance is low. To cope therewith, the three-dimensional wavelet coding has been developed for performing a subband coding for each frame after performing a time filtering, which accompanies the motion compensation in a spatial region, for the raw image to remove the time-directional correlation.
Hereinafter, a technology of the typical three-dimensional wavelet coding will be explained (for example, non-patent document 1).
Each of FIG. 18 to FIG. 21 is a view for explaining the three-dimensional wavelet coding shown in the non-patent document 1. FIG. 18 is a block diagram illustrating a configuration of a coding unit in the three-dimensional wavelet coding. Hereinafter, a flow of the process of the three-dimensional wavelet coding will be explained by employing FIG. 18.
A time-directional filtering section 201 performs a wavelet transform accompanying the motion compensation in the time direction for input image signals 2000, which are comprised of N sheets (N is the power of 2) of consecutive frames, thereby to generate N/2 sheets of time low-band subband signals 2001, N/2 sheets of time high-band subband signals 2002 and movement information 2003. The time-directional filtering in the time-directional filtering section 201 is recursively performed for the time low-band subband signals 2001 out of them. The subband division is performed in the spatial direction for one sheet of the time low-band subband signals 2004 and (N−1) sheets of the time high-band subband signals 2002 generated in the multi-staged time-filtering process, respectively.
A spatial sunband divider 202 horizontally and vertically performs a subband division for the time high-band subband signal 2002 into two respectively, and generates one time high-band/space low-band subband signal 2005 and three time high-band/space high-band subband signals 2006. The time high-band/space low-band sunband-signal 2005 out of them is recursively divided by the spatial subband divider 202. After performing a subband division in the spatial direction only the specified number of times in such a manner, the time high-band/space low-band subband signal 2005 and the time high-band/space high-band subband signal 2006 are quantized by quantizer 204.
The spatial subband division is performed for a time low-band subband signal 2004 as well in a multi-stage by a spatial subband divider 203, and a time low-band/space low-band subband signal 2007 and a time low-band/space high-band subband signal 2008 are quantized by a quantizer 204. Respective quantized signals are entropy-coded by an entropy coder 205.
In addition hereto, a movement-information coder 206 codes the movement information 2003 generated by the time filtering section 201. Respective coded signals, which are multiplexed by a multiplexer 207, are output as a bitstream 2010.
FIG. 19 is a conceptual view illustrating the subband division in the high-order time direction and spatial direction in the three-dimensional wavelet coding. An input signal 2011 is divided into a tertiary time low-band subband signal 2015 and a tertiary time high-band subband signal 2016, a secondary time high-band subband signal 2014, and a primary time low-band subband signal 2012 and a primary time high-band subband signal 2013 with the three-staged time-filtering.
The tertiary time low-band subband signal 2015 is divided into a tertiary time low-band/space low-band subband signal 2017, tertiary time low-band/space high-band subband signals 2018, 2019, and 2020, and secondary time low-band/space high-band subband signals 2021, 2022, and 2023, and primary time low-band/space high-band subband signals 2024, 2025, and 2026 with the three-staged spatial subband division.
The time high-band subband signal 2016 is divided into a tertiary time high-band/space low-band subband signal 2027, tertiary time high-band/space high-band subband signals 2028, 2029, and 2030, and secondary time high-band/space high-band subband signals 2031, 2032, and 2033, and primary time high-band/space high-band subband signals 2034, 2035, and 2036 with the three-staged spatial subband division.
In a case of reconfiguring the image signal having a spatial resolution or a frame rate, which is different from that of the input image signal, from the bitstream, the decoding unit decodes only one part out of coded data of a plurality of subband signals that are contained in the bitstream.
The process of extracting the coded data of the subband signals will be explained according to FIG. 19.
So as to reconfigure the moving image of which the frame rate is half, the decoding unit decodes the coded data that corresponds to the time low-band subband signal 2015 and the time high-band subband signals 2016 and 2014. So as to reconfigure the moving image of which the resolution is half, the decoding unit decodes 2023 from the subband signals except the primary time low-band/space high-band subband signal, i.e., the time low-band/space low-band subband signal 2017 and the time low-band/space high-band subband signal 2018, out of the time low-band subband signals.
In addition hereto, the decoding unit decodes 2033 from the subband signals except the primary time high-band/space high-band subband signal, i.e., the time high-band/space low-band subband signal 2027 and the time high-band/space high-band subband signal 2028, out of the time high-band sunband signals.
FIG. 20 is a block diagram illustrating configurations of the coded-data extracting apparatus for extracting the coded data that corresponds to the reduced image from the bitstream generated with the three-dimensional wavelet coding, and the moving image decoding apparatus.
The coded-data extracting apparatus discards a space high-band subband signal 2038 of which the order is lower than that of a bitstream 2010, generates a bitstream 2037 that is comprised of the coded data of the suitable subband signal, and transmits it to a moving image decoding apparatus 209. The moving image decoding apparatus 209 synthesizes the subband signals that are contained in the bitstream 2037, and outputs a decoded-image signal 2047.
FIG. 21 is a block diagram illustrating a configuration of the moving image decoding apparatus 209. A flow of the decoding process in the three-dimensional wavelet decoding will be explained by employing FIG. 21.
An inverse multiplexer 210 cuts out the coded data of the subband signal from the bitstream 2037, and generates a time high-band/space high-band subband signal 2039, a time high-band/space low-band subband signal 2040, a time low-band/space high-band subband signal 2041, and a time low-band/space low-band subband signal 2042 through an entropy decoder 211 and an inverse quantizer 212.
A spatial subband synthesizer 213 performs a subband synthesis for the time high-band/space high-band subband signal 2039 and the time high-band/space low-band subband signal 2040 recursively, and generates a time high-band subband signal 2043.
A spatial subband synthesizer 214 performs a subband synthesis for the time low-band/space high-band subband signal 2041 and the time low-band/space low-band subband signal 2042 recursively, and generates a time low-band subband signal 2044. Herein, the number of times of the spatial subband synthesizing process, which is smaller than that of the spatial subband dividing process performed in the coding unit, is decided by the space high-band subband signal discarded by the coded-data extracting apparatus 208.
In addition hereto, a motion-information decoder 215 decodes the motion information output by the inverse multiplexer 210, and generates movement information 2045. A vector reducer 216 reduces a vector length of the motion information 2045 according to a resolution ratio of the input signal at the time of coding to the decoded image signal that is output from the decoding unit. This ratio is decided by the number of the space high-band subband signal discarded by the extracting apparatus 208. For example, in a cased where the space high-band subband signal of which the order is lowest has been discarded, the vector length is reduced to a half.
Thereafter, a time-directional inverse-filtering section 217 performs an inverse transform of the time filtering at the time of coding for the time high-band subband signal 2043 and the time low-band subband signal 2044 according to a motion information 2046 output by the vector reducer 216, and generates a decoded signal 2047.    [Non-patent document 1] J.-R. Ohm, “Three-dimensional subband coding with motion compensation”, IEEE Trans. Image Processing, vol. 3, pp. 559-571, September 1999